Abstract
Isolation of high-level radioactive waste (HLW) in deep geological repositories (DGR) through a multibarrier concept is the most accepted approach to ensure long-term safety. Clay minerals are one of the most promising materials to be used as engineered barriers. In particular, high charge micas, as components of the engineered barrier, show superselectivity for some radioactive isotopes and a large adsorption capacity, which is almost twice that of the other low charge aluminosilicates. In addition, high charge micas are optimum candidates for decontamination of nuclear waste through two different mechanisms; namely an ion exchange reaction and a nonreversible mechanism involving the formation of new stable crystalline phases under hydrothermal conditions. In this work, we report a new in situ optical sensor based on the incorporation of Eu3+ in these high charge micas for tracking the long-term physical-chemical behavior of HLW contaminants in DRG under mild hydrothermal conditions. The incorporation of Eu3+ into the interlayer space of the mica originates a well resolved green and red luminescence, from both the 5D1 and 5D0 excited states, respectively. The formation of new crystalline phases under hydrothermal conditions involves important changes in the Eu3+ emission spectra and lifetime. The most interesting features of Eu3+ luminescence to be used as an optical sensor are (1) the presence or absence of the Eu3+ green emission from the 5D1 excited state, (2) the energy shift of the 5D0 → 7F0 transition, (3) the crystal-field splitting of the 7F1 Eu3+ level, and (4) the observed luminescence lifetimes, which are directly related to the interaction mechanisms between the lanthanide ions and the silicate network.
Highlights
High-level radioactive waste (HLW) requires a great level of isolation from the environment to assure long-term storage
In this paper we report a new in situ optical sensor based on the incorporation of Eu3+ in these high charge micas for tracking the long-term physicalchemical behaviour of HLW contaminants in DRG under mild hydrothermal conditions
We present a new in situ optical sensor based on the incorporation of Eu3+ in a high charge mica with 2 negative charges per unit cell, hereafter Mica-2, for tracking the long-term physicalchemical behaviour of HLW contaminants in deep geological repositories (DGR) under mild hydrothermal conditions
Summary
High-level radioactive waste (HLW) requires a great level of isolation from the environment to assure long-term storage. The more localized negative charge in the vicinity of the silicon atom due to the proximity of tetrahedral aluminum, the higher electrostatic interaction between these ions and the interlayer rare-earth cations.[16] High charge micas are trioctahedral clays with a Si:Al ratio in the tetrahedral sheet from 3:1 to 1:1 This particular structural feature makes this family of synthetic clays an optimum candidate for decontamination of nuclear waste through an ion exchange reaction but mainly through the formation of new crystalline phases.[9]. Eu3+ sharp-line luminescence provides valuable information concerning the crystalline structure of the host material
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